1. Field of the Invention
The present invention generally relates to a motor protecting apparatus, and in particular to a current supply control apparatus having an overcurrent protective function of protecting an electrically-powered motor and a drive control circuit therefor with suppression of overcurrent in an electric vehicle.
2. Description of the Prior Art
In recent years, engineers rapidly pursue development of electrically-powered motor vehicles, and particularly demand for electric cars including hybrid cars is rapidly increased for the purpose of environmental protection measures. Either a DC motor or an AC motor is used as a driving power unit of this type of electric vehicles. In particular, a synchronous motor using permanent magnet for a rotor of the motor is mainly developed for an electric car because such a synchronous motor exhibits high efficiency.
Conventionally, synchronous motors of this type are widely used in a factory automation field in industry, and they are also suited for use in a field of electric cars while the basic control methods of the synchronous motors of this type are essentially the same both in the factory automation field and the electric vehicle field.
In a conventional control method of a synchronous motor for use in a factory automation field, when an overcurrent of a drive power device for driving a power motor is detected, electric current supply to the motor is instantaneously cut off to control the overcurrent of the power device so that the current supply to the motor is completely and immediately cut off.
However, in the case of an electric car which is driven using an electrically-powered motor, if the motor is suddenly controlled in the same manner as in the factory automation, there may cause a danger that the electric car may suddenly and unexpectedly stop and a behind car may collide, and further when a front car suddenly stops, it is impossible to avoid a front danger. On the other hand, when an overcurrent is detected, failure to interrupt the current supply may result in serious damage to the motor power device such as a fire occurrence of a motor drive unit and the like control circuit in the electric car.
In order to avoid such a problem involved in the conventional electrically-powered motor vehicles, in the disclosure of U.S. Pat. No. 5,592,355, there is suggested a motor control circuit which provides for improved ON/OFF switch safety in an electric car operation by temporarily interrupting a motor power supply as a means of eliminating an overcurrent supply to the motor when an overcurrent is detected, and in the case where a period integrating the overcurrent detection time exceeds a predetermined time limit, the motor power supply is completely shut out.
In this construction, even when an overcurrent is detected, the current supply is not completely stopped but merely temporarily interrupted, and the current supply for motor drive is retrieved when the current supply is appropriately reduced by eliminating the overcurrent. In this construction, however, when the ON/OFF switch of electric current supply to the motor is frequently repeated, a variation in torque of the motor is increased, which is offensive to a driver and passengers.